The footage is Red 4K footage transcoded to ProRes 422 HQ at 2K; there is also some 5D footage transcoded to ProRes 422 HQ at 1080p.

They have a Kona 3 card in the Mac Pro at the moment w/o the Kona software/drivers installed. They also have the breakout box.

They are not using a professional monitor (i.e. with SDI or HD-SDI) and they are not ingesting from or outputting to any decks.

My question is this - is the Kona card of any benefit to us (assuming we install the drivers)? Will it help ease the burden of playback on the CPU? Is the breakout box of any use to us? The only thing I can see that the card and breakout box gain for us is better audio outputs for monitoring (the breakout box has RCA out) - we are currently picking up audible radio station interference through the speakers.

No benefit. THe card it to help getting video into and out of your system. If you aren't capturing from tape, outputting to tape or a monitor, then it is useless. It doesn't do anything to help with playback of the footage just inside the system.